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Should MMU2S Pulley Motor Move While Printing?  

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Andre Campos
(@andre-campos)
New Member
Should MMU2S Pulley Motor Move While Printing?

I have upgraded my Mk3 to the MK3S + MMU2S combo several months ago, and beyond the scope of this question, I haven't been able to get much "MMU" prints out of the setup, so mostly I've been still using using it for single color prints.

In one of my periodic attempts to make the MMU work, I recently took some time to re-align the PET tubes from the filament buffer to the printer, not sure if this is the root cause for my issues, but now even printing with a single filament is not working. The print starts, I get part of the first layer, then it looks like the flow of filament drops causing the print to fail. 

After attempting to clean things up and ensure that everything was fitted properly I was finally able to get a good print when I bypassed the MMU and feed the filament directly to the extruder, so I'm rather confident that my issue is within the MMU setup. 

I've also noticed that after the MMU loads the filament at the beginning of the print, the pully motors don't turn anymore, and this was unexpected for me. I expected that it would turn at the same rate as the extruder so that it would help feed filament and overcome the additional friction given the larger path it has to go through because of the MMU. I can manually pull the filament in and out (withoyt having to put too much force into it) of the MMU output so I'm under the impression that the selector is not gripping the filament after the load operation is completed. 

So I would like to check if this has always been like this, and if anyone has any suggestions / recommendations on how to get things up and running again. 

Posted : 10/03/2020 8:29 pm
vintagepc
(@vintagepc)
Member
RE: Should MMU2S Pulley Motor Move While Printing?

Yes, this is normal. MMU is not engaged once the extruder takes over.

Posted : 10/03/2020 8:54 pm
dyits
(@dyits)
Active Member
RE: Should MMU2S Pulley Motor Move While Printing?

I also have same issue, and hope mmu motor also trun when priting, why prusa did edit the frimware like this?

Posted : 23/03/2020 6:26 am
vintagepc
(@vintagepc)
Member
RE: Should MMU2S Pulley Motor Move While Printing?

Trying to keep two motors engaged and syncronized without either grinding the filament or overfeeding the rear drive is a very challenging task, especially given the way the MMU communicates with the printer. 

Bottom line is it would massively complicate the MMU even further. And I don't think anybody wants that.

 

Posted : 23/03/2020 1:40 pm
Andre Campos
(@andre-campos)
New Member
Topic starter answered:
RE: Should MMU2S Pulley Motor Move While Printing?

Makes sense vintagepc, attempting to keep both of them locked and in sync would make the mechanism even more complex, but the longer path and transitions introduce a lot of friction and hotspots that the filament can get pinched against. 

I printed this part: https://www.thingiverse.com/thing:3233579 in a attempt to reduce any resistance from the filament entering the MMU, and also since I'm basically just attempting to print with a single color for now, I'm feeding directly from the spool to the MMU (still have the PET tube connected to the MMU input) but I still have something along the way that is holding the filament causing the extruder to eventually stop after a few minutes of print time. 

What I don't understand is that if I remove the PET tube from the output of the MMU, I can manually move it without any major noticeable resistance. 

Posted : 23/03/2020 3:53 pm
vintagepc
(@vintagepc)
Member
RE: Should MMU2S Pulley Motor Move While Printing?

I've found the stock PTFE tubes can cause issues if your filament is oversize or has a rough surface finish (e.g. AmazonBasics). 

 

I ditched the buffer setup entirely, opting for auto-rewinders and swapped the spool-side PTFEs for some that are 3mm ID. It helped a ton. 

Posted : 23/03/2020 5:05 pm
dyits
(@dyits)
Active Member
RE: Should MMU2S Pulley Motor Move While Printing?

I find most friction comes form the tube between mmu and extruder. and I even try 4 mm tube, but it feels no change.

especially, when print part is a little tall. the tube bent more.

Posted : 24/03/2020 1:41 am
dyits
(@dyits)
Active Member
RE: Should MMU2S Pulley Motor Move While Printing?

@vintagepc

if some one who have orginal prgramme of printer and mmu, maybe I can try

Posted : 24/03/2020 1:43 am
dyits
(@dyits)
Active Member
RE: Should MMU2S Pulley Motor Move While Printing?

@carlos-andre-t

Good idea, I also try a lot of ideas. and find most friction  is from tube between extruder and mmu. 

but if use mmu , the tube can not be cancelled. 

Posted : 24/03/2020 1:47 am
vintagepc
(@vintagepc)
Member
RE: Should MMU2S Pulley Motor Move While Printing?
Posted by: @dyits

@vintagepc

if some one who have orginal prgramme of printer and mmu, maybe I can try

It's all on Prusa's github, go to town.

Posted : 24/03/2020 11:28 am
vintagepc
(@vintagepc)
Member
RE: Should MMU2S Pulley Motor Move While Printing?
Posted by: @dyits

I find most friction comes form the tube between mmu and extruder. and I even try 4 mm tube, but it feels no change.

especially, when print part is a little tall. the tube bent more.

I doubt that. The stock tube has 4mm OD and all parts are designed for that, so you cannot have had 4mm ID tubing. 

 

 

Posted : 24/03/2020 11:43 am
Andre Campos
(@andre-campos)
New Member
Topic starter answered:
RE: Should MMU2S Pulley Motor Move While Printing?

@dyits

Interesting, perhaps passing the tube though a spring near the extruder and near the output of the MMU can help reduce the bend radius near these critical points and reduce the friction. Has anyone tried this strategy before? 

Posted : 24/03/2020 3:54 pm
dyits
(@dyits)
Active Member
RE: Should MMU2S Pulley Motor Move While Printing?

@carlos-andre-t

I have  tried use a mental spring tube  to replace PETE tube, but still same. 

Posted : 27/03/2020 1:28 pm
pcweber
(@pcweber)
Member
RE: Should MMU2S Pulley Motor Move While Printing?

I posted yesterday this. https://forum.prusa3d.com/forum/original-prusa-i3-mmu2s-mmu2-hardware-firmware-and-software-help/mmu2s-idler-problems/

since I was having several problems that I had chalked up to friction. That wasn't the case, as a matter of fact, when I first mounted the mmu, I had to take the blade off of the selector because it was cutting into parts rather than the filament.

Today I took advantage of the upgrade to the selector, which supposedly will make the cutter work better, to replace the parts that were damaged by the idler and cutter combo. I verified zero drag on the filament as I put it all back together and there is absolutely no drag from the buffer to the extruder.

 

Phil

Stay safe and healthy, Phil

Posted : 01/04/2020 5:02 pm
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